Which credit card should I use for this purchase?

Picking a card at a till is not a matter of taste. Four facts decide it, three of them are printed on the issuer's own page, and the fourth is one only you know.

Fact zero, which outranks the other three

Are you clearing the statement balance in full?

If the answer is no, stop optimising rewards. Interest is charged on what you owe, every day, at a rate well above any cashback percentage, and it keeps being charged for as long as the balance exists. A reward is a percentage of one purchase, collected once.

On a card carrying a balance, the correct card is the cheapest one to borrow on, and the rewards are not part of the decision. Everything below assumes you pay in full.

Fact one: the rate for that specific category

A card has a shape, not a number. The Chase Freedom Unlimited pays 1.5% on all other purchases, 3% on dining at restaurants including takeout and eligible delivery, and 3% on drugstore purchases. ³ That card is a good answer in a restaurant and a mediocre answer at a petrol pump.

Flat-rate cards have a different shape. The Citi Double Cash Card pays unlimited 2% cash back on purchases, and the Wells Fargo Active Cash Card pays unlimited 2% cash rewards on purchases. ^{2 4} Neither will ever be the best card for any category, and both will beat a category card everywhere outside its categories.

Fact two: the cap on that rate

This is the fact most advice leaves out, and it is the one that moves during the year.

The American Express Blue Cash Preferred Card pays 6% cash back at US supermarkets on up to $6,000 per year in purchases, then 1%. ¹ The Blue Cash Everyday Card pays 3% at US supermarkets on the same $6,000 annual cap, then 1%. ⁵

Work the consequence through on the Preferred card. Under the cap, it earns 6% on groceries against a flat card's 2%, so it is comfortably ahead. Above the cap, it earns 1% while the flat card still earns 2%, so the flat card earns twice as much on every further pound of groceries that year. The card did not change. Your position within its cap did.

Fact three: the annual fee, at your spending

The Blue Cash Preferred Card carries a $95 annual fee after a $0 introductory year. ¹ The Blue Cash Everyday Card has no annual fee. ⁵

The comparison is not which fee is smaller. It is how much category spending it takes for the extra rate to cover the extra fee.

Against a flat 2% card, the Preferred card earns 4 extra points of rate on supermarket spending under the cap. $95 divided by 0.04 is $2,375 of supermarket spending a year before the fee is repaid. Below that, the no-fee card wins; above it and under the $6,000 cap, the fee card wins. That number is yours, not a reviewer's. It falls out of two published figures and one fact about your own shopping.

The order to check, in practice

1. Am I paying this card off in full, and if not, which of my cards is cheapest to borrow on?
2. What category is this purchase, as the card issuer defines it rather than as the shop describes itself?
3. Which of my cards has the best published rate for that category?
4. Has that card already spent its cap for the period?
5. Does the annual fee on the winner still make sense at my real spending?

Steps 2 and 4 are where mistakes happen. A supermarket petrol station is usually coded as fuel, not groceries. A cap spent in February is still spent in November.

Where CardLab helps

The awkward part of this is not the arithmetic. It is that steps 3 and 4 need every card's published rate and your own running spend against every cap, at the same time, at a till.

CardLab's Best Card ranks the cards you hold for a chosen purchase or purpose, and shows the comparison behind the ranking rather than just a winner, including the point at which the order would change. It tracks spend against each card's cap and drops a card down the ranking when the cap is spent, then says that is the reason. Every rate and fee it uses stores the issuer page it was read from, the exact sentence on that page, and the date it was read, so you can check any answer against the source.

Two constraints are worth stating plainly. The ranking is offered only in the 17 markets that have a verified catalog; elsewhere the tracker and simulators still work, because they compute from the terms you enter yourself. And CardLab takes no commission from any issuer, so no card is ranked or featured because of a payment.